The present invention relates to a novel face coat composition for a mold for casting of a metal having a relatively high melting point as well as to a method for forming a face coat layer of a casting mold.
A lost-wax mold for casting of a metal or alloy having a relatively high melting point such as titanium and titanium-based alloys has a layered structure consisting of a so-called face coat layer on the surface of the mold cavity coming into contact with the melt and a backup coat layer by which the general configuration of the mold is formed with reinforcement as a backup of the face coat layer. A face coat layer of a casting mold is formed by coating a wax-made model of the desired cast body with a slurried composition containing a refractory material to form a layer followed by drying and formation of the backup coat layer thereon, removal of the wax by melting under heating and finally calcination of the double-layered casting mold.
It is desirable that the material for the face coat layer coming into contact with the high temperature melt in the course of casting is as highly refractory as possible. Conventional refractory materials used as a material of such a face coat include high melting-point metals such as tungsten and molybdenum and oxides such as zirconium oxide, calcium oxide and the like. Use of a high melting-point metal as the refractory material in the face coat composition is accompanied by an industrially difficult problem that, since hydrogen reduction must be undertaken in the firing step of the mold before casting, large-scale facilities are required for the preparation of the casting mold if not to mention the complicacy of the control of the process. When zirconium oxide is used as the face coat material among oxides used for the purpose, a problem must be solved that zirconium oxide has reactivity with melts of certain metals to form a reaction-hardened layer on the surface of the cast body which must be removed by taking a large amount of labor and time in order to finish the cast article which eventually cannot have a good dimensional accuracy as a consequence. Calcium oxide as a material of the face coat layer has a problem that calcium oxide is highly hygroscopic so that difficulties are encountered in the preparation of the casting mold and subsequent handling thereof.
Alternatively, a proposal has been made, for example, by D. R. Schulyer et al. in Proceedings of Vacuum Metallurgy Conference, edited by R. C. Krutenat, pages 475-503 (1977) for the use of yttrium oxide which is little reactive with a high melting-point, active metal or alloy such as certain titanium-based alloys by using an aqueous colloidal liquid of potassium silicate with fine particles of yttrium oxide dispersed therein with further admixture of relatively coarse particles of yttrium oxide in the formation of a face coat layer of a casting mold. As is reported by Schulyer, however the face coat layer thus formed by using the above mentioned face coat composition has a surface not smooth enough as compared with the conventional face-coated molds prepared by the workmanship of an ordinary level having air bubbles and pits as well as many stucco particles appearing from part to part.
Further, Japanese Patent Kokai No. 62-143864 discloses a face coat composition which is an aqueous slurry by using a yttrium oxide sol as a binder. This patent document also mentions on the use of a zirconium oxide sol instead of the yttrium oxide sol. According to the results of the evaluation tests given there, the face-coated casting molds prepared by using such a face coat composition could be highly evaluated with a relatively small thickness of the reaction-hardened layer on the surface of the cast bodies prepared by using the mold. A problem in this method, however, is that the stability of the face coat composition in the form of a slurry is questionable because yttrium oxide sols are inherently unstable.
Japanese Patent Kokai No. 63-115644, on the other hand, discloses a face coat composition in the form of a slurry containing yttrium oxide with alcohol as the dispersing medium and a partial hydrolyzate of ethyl silicate as the binder. A comparison of this face coat composition is given there with a similar composition prepared by using water as the dispersing medium in place of the alcohol. According to this patent document, the face coat composition is acceptable in respect of the relatively small thickness of the reaction-hardened layer on the surface of the cast bodies prepared by using a mold having a face coat layer thereof. This advantage, in fact, could be confirmed by the inventors' experiments repeating the disclosure. A serious problem in this method, however, is also the instability of the face coat composition. Namely, the slurried face coat composition has an unduly short pot life not suitable for practical use. For example, coagulation of the slurried face coat composition proceeds already within 12 hours from preparation when the weight proportion of the yttrium oxide in the composition to the silicon dioxide exceeds 7.5% and within a few days even when the proportion is lower than that.
Further, when a mold prepared from the above described face coat composition is used for casting of a high melting-point, active metal such as titanium or a titanium-based alloy, the silicon dioxide constituent in the slurry of the face coat composition reacts with the melt of the metal or alloy under casting so as to form a hard but brittle reaction-hardened layer, which must be removed to finish the cast body by chemical milling and grinding or polishing unavoidably resulting in a decreased dimensional accuracy of the cast body.
In addition to the above described problems, following problems are involved in the use of yttrium oxide as a refractory constituent in the face coat composition.
It is necessary in the use of yttrium oxide, namely, that the temperature of calcination or firing of the casting mold must be increased to 1600.degree. C. or higher in order to impart the mold with a strength capable of withstanding the attack of the flow of the melt which otherwise destroys or deforms the mold while the refractory materials such as zircon- or mullite-based ones usually used as the material of the backup coat have such refractoriness as not to withstand a temperature of firing of the mold exceeding 1100.degree. C. so that, when firing of the mold is conducted at an excessively high temperature, the refractory material of the backup coat layer is subject to softening and the mold is eventually broken. Accordingly, the use of yttrium oxide as the refractory constituent in the face coat composition is not in compliance with the use of a zircon- or mullite-based refractory material for the backup coat layer.
On the other hand, a proposal is made in Japanese Patent Kokai No. 3-8534 for the use of an aqueous slurry containing yttrium oxide which is admixed with a water-soluble organic polymer as the binder but the green strength of the face coat layer formed therefrom is so low that, in the step of dewaxing as one of the steps in the lost-wax casting method, the face coat layer is eventually destroyed by being lost with the molten wax.